Magnetized Collisionless Shock Formation Mediated by the Modified Two-Stream Instability

Two-dimensional particle-in-cell simulations are used to study the feasibility of using terawatt laser systems to form perpendicular magnetized collisionless shocks in hydrogen and neon plasmas. With experimentally accessible parameters, shocks can form within a few tenths of a nanosecond. Ions are reflected from the shock front, indicating that these shocks reach supercriticality. A modified two-stream instability (MTSI) from the incoming and reflected ions is shown to be the operating microinstability mediating shock formation, and the shock formation time and shock width are determined by MTSI. With the realistic ion/electron mass ratios used, the MTSI growth rates are much larger than the ion gyrofrequencies. These perpendicular collisionless shocks form within approximately a tenth of an ion gyroperiod. Mode analysis in the shock transition further confirms MTSI is the operating instability.